The KC-97 Stratotanker was an aerial refueling tanker variant of the C-97 Stratofreighter that was greatly modified with all the necessary tanks, plumbing, and "flying boom." The cavernous upper deck was capable of accommodating oversize cargo accessed through a very large left-side door while transferable jet fuel was contained in tanks on the lower deck. Both decks were heated and pressurized for high altitude operations. It was the backbone of the United States Air Force's tanker fleet until replaced by the Boeing KC-135.